An embodiment of the invention is directed to a transformer architecture coupled to a port of a radio frequency (RF) active circuit to improve common stability.
Active RF circuits such as amplifiers and mixers often have a balanced (or differential) input stage. In that case, the input signal has two components, which are simultaneously applied to two similar terminals of the input stage, respectively. The active circuit in effect operates upon the difference between the two components, which tends to inherently cancel any noise signals that are essentially identical in phase and amplitude and that appear simultaneously on both terminals. This is referred to as common mode noise. The balanced input stage allows the active circuit to reduce the impact of common mode noise present at the input.
When an active RF circuit that has a differential input is to be driven by a single-ended source, a passive transformer referred to as a balun is used, to convert the single-ended signal (unbalanced) into a differential one (balanced). If the frequencies of operation are sufficiently high, such as in the microwave region and beyond, this arrangement may create a resonance that produces an undesired, common mode oscillation at the input to the active circuit. For instance, consider a mixer in a balanced configuration, which may have a balanced input port, a balanced output port, and a balanced local oscillator (LO) port. If the LO port has no differential signal applied to it, then that port should not exhibit any substantial signal. However in many cases, and particularly where the mixer is designed to operate with LO differential signals at 30 GHz or higher, a pair of essentially similar oscillatory signals appear on the respective input terminals of the LO port spontaneously. This is referred to as common mode oscillation.